1. Field of the Invention
The present invention relates to an image reading apparatus (e.g., scanner, copying machine, facsimile) which includes a control unit that operates based on a frequency-spread clock signal, and to a reading control method.
2. Description of the Related Art
Various types of image reading apparatus have been proposed for scanning, copying and/or faxing documents, including a type of image reading apparatus in which a document placed on a document positioning plate is scanned by a line sensor, and another type of image reading apparatus which reads the document by feeding the document to a fixed line sensor.
In addition, a complementary metal-oxide semiconductor (CMOS) sensor, a charge-coupled device (CCD) sensor or the like may be used as a reading unit (reading device). Moreover, there are a number of types of light sources and methods for controlling a light source.
The image reading apparatus may include a circuit board which has a control circuit and a memory and the like. The control circuit performs various image processing operations such as drive control of the reading device, capturing the image data which has been read by the reading device and the like. In addition to this circuit board, the image reading apparatus is provided with another circuit board mounting a reading sensor for scanning a document. These two circuit boards are normally connected with each other through a flexible flat cable (FFC), harness or the like. Several driving signals are necessary for driving the reading unit, and they are supplied to the reading unit via a cable.
The above-mentioned image reading apparatus may not be capable of avoiding electromagnetic radiation (EMR) or electromagnetic interference (EMI) from the circuit board mounting the reading unit, a cable or the like. Further, if a faster operation of the reading unit is desired, the frequency of the driving signal in the reading unit may need to increase. As a result of increasing the operation speed of the reading unit, the level of EMR emitted from the image reading apparatus may become higher.
In order to reduce the level of EMR emitted by an image reading apparatus, for example, as in the above-mentioned situation, Japanese Patent Application Laid-Open No. 2002-281252 discusses the spectrum spread clock generating circuit provided in the reference clock signal generating portion. The reference clock signal is supplied to the control circuit and the memory. The spectrum spread clock generating circuit is referred to as a Spectrum Spread Clock Generator (SSCG), for example. The above-mentioned EMR is often a higher harmonic wave, which is the multiplied reference clock signal. Accordingly, it is possible to reduce the peak level of EMR by modulating the frequency of the reference clock with the SSCG. FIG. 8A is a view illustrating the modulation between frequencies fL and fH. The period of the modulation is represented by T.
However, when an analog image signal obtained in the reading unit is processed based on the modulated reference clock signal, deviation in the timing due to the modulation may occur.
FIG. 8B is a view illustrating the timing at which an analog signal S1 is sampled based on the clock signal. Sp represents a sampling signal. The frequency of a reference signal of the sampling signal Sp is also modulated. Accordingly, there is a deviation of a period ΔT between the sampling timing in the maximum frequency and that in the minimum frequency.
FIG. 9A is a view illustrating a phase relation between a timing signal of the CCD and frequency modulation in case where a reading operation is performed on a line-by-line basis by the CCD. The CCD is driven in synchronization with a signal SH which is output on a line-by-line basis. When an image is read out, a reading unit is moved at a constant speed. At this time, a signal SH is output at an interval corresponding to the moving speed.
Hereinafter, for ease of explanation, the reading of four lines from nth line to (n+3)th line and the phase of the first timing signal in each line are described. f91 represents the phase relation between the first timing signal of the nth line and the frequency modulation. The modulation phase from 0 to 180 degrees (π) is illustrated. f92 represents the phase relation between the first timing signal of (n+1)th line and the frequency modulation. Likewise, f93 and f94 represent similar relations in an (n+2)th line and in an (n+3)th line, respectively.
In this case, since the timing of the phase of the modulation is not controlled, a constant phase difference exists between the lines caused by the period of the signal SH and output timings of timing signals φ1 and φ2. In FIG. 9A, the phase difference of about T/4 exists between f91 and f92. The phase difference of about T/4 exists also between f92 and f93. As described above, the constant phase difference exists between the lines. While the first timing signal is described in this case, the second pulse and the successive pulses also have the similar relation because the timing signals φ1 and φ2 are output at a constant period.
Therefore, even if the document having a constant density is read over a plurality of lines, pseudo streaks can be recognized when the image is displayed on a monitor. These pseudo streaks are described in a schematic diagram illustrated in FIG. 9B. “A” represents a scanning direction, and “B”represents a direction in which the line sensor moves from a first line to an eighth line. In FIG. 9B, streaks represented by St appear in an oblique direction. These streaks illustrate the constant phase of the clock signal between lines.
In order to obscure the streaks in the read out image, according to Japanese Patent Application Laid-Open No. 2002-281252, a synchronization signal period of the main scanning is synchronized with a period of the frequency spread of the SSCG. Therefore, the SSCG is configured such that the same phase of a SSCG modulation signal is generated at a certain position from a reference point of the main scanning. In such a configuration, the positions of the streaks appearing in the image are fixed and the streaks are obscured.
However, when the SSCG is configured of the analog circuit, the frequency modulation period itself is often not maintained constant, which is caused by variation in a manufacturing process and a temperature characteristic. Therefore, it is difficult to stably control the phase and period of main scanning synchronization signal and SSCG modulation signal depending on the characteristics of the SSCG.